A metallic element found in certain rare minerals, as thorite, pyrochlore, monazite, etc., and isolated as an infusible gray metallic powder which burns in the air and forms thoria; -- formerly called also thorinum. Symbol Th. Atomic weight 232.0.

An element that is a byproduct of the decay of uranium.

See table. ... : an isotope of polonium produced in thorium decay, namely 216Po (half-life = 0.15 s).[ Soddy] ... : The names of two radioisotopes, both produced in thorium decay, included thorium C. Simple thorium C was an isotope of bismuth, namely 212Bi (half-life = 61 min); thorium C' was an isotope of polonium, namely 212Po (half-life = 0.3 µs) [ Rutherford, Soddy] ... : an isotope of thallium produced in thorium decay, namely 208Tl (half-life = 3 min). [Soddy & ] ... : an isotope of radium produced in thorium decay, namely 224Ra (half-life = 3.6 d). [ Rutherford, Soddy